Geography Reference
In-Depth Information
scheme which causes confusion. While there are too many exam-
ples to describe in this topic some names from the US Department
of Agriculture system are: vertisols (swelling clay soils with deep,
wide cracks), histosols (organic rich soils), andisols (formed on vol-
canic parent material, especially ash) and oxisols (red/yellow/grey
soils of tropical and subtropical regions with strongly weathered
horizons enriched in silica, clay and oxides of aluminium and iron
and are acidic with low nutrient status).
Physical properties of soils
Soil texture and structure influence how soils work and how they
can be managed. Soil texture refers to the relative proportions of
sand-, silt- and clay-sized materials in a soil. Clay particles are
smaller than 2 micrometres (two-millionths of a metre), silt is
between 2 and 60 micrometres while sand is between 60 and 2,000
micrometres in diameter. The texture controls the water holding
capacity, aeration, drainage rate, organic matter decomposition
rates, compaction, susceptibility to water erosion, ability to hold
nutrients and leaching of pollutants. Figure 3.4 shows how soil
texture classifications are based on the relative proportion of each
particle size. For example, reading the figure shows that if a soil is
40 per cent sand, 30 per cent silt and 30 per cent clay it would be
classified as a clay loam.
Good soil structure is important for achieving well-drained and
aerated soils. Soil structure refers to the arrangement of soil parti-
cles. Soil particles usually attach themselves to each other in forma-
tions called
peds
. Where a soil lacks peds, such as in a sand dune,
the soil is described as structureless. Soil structure is characterised in
terms of the shape, size and distinctness of these peds with four
principal types: blocky (roughly equally sized on each side, almost
cube-shaped, but the peds can be angular or more rounded), sphe-
roidal (sphere-shaped), platy (horizontal plates) and prismatic (ver-
tically elongated columns of soil with flat face). Clay particles and
organic compounds largely hold peds together. As a result, coarse-
textured soils tend to have weakly developed structures, whereas
ine-textured soils generally have moderate to strong structures.
The strength of peds influences resistance to erosion and the ease
of cultivation.
